Carbon dioxide has been known for centuries, with Pliny the Elder describing it in the context of poisonous vapors coming from caverns. In the seventeenth century, Van Helmont obtained carbon dioxide by such means as fermentation and acidification of carbonates, and also studied many of its properties. Later it was recognized as an acidic gas but it was not until the end of the eighteenth century that Lavoisier recognized it as a compound of carbon and oxygen of a given proportion.
Since mineral waters (solutions of carbon dioxide in water) were thought to have medicinal properties, there was from the onset an incentive to commercially exploit carbon dioxide. Farady made liquid and solid carbon dioxide using a hydraulic pump and studied solid carbon dioxide as a refrigerant. Its uses over time has proliferated to include such diverse applications as beverage carbonation, chemical manufacture, fire fighting, food freezing, greenhouses, oil well secondary recovery, as an atmosphere in welding, and even more recently in supercritical extraction processes.
The bulk of carbon dioxide is generated from ammonia and hydrogen plants as process gas carbon dioxide resulting from the reaction between hydrocarbons and steam. The carbon dioxide produced by such methods has a high purity but may contain, inter alia traces of hydrogen sulfide, sulfur dioxide, and hydrocarbons which are particularly detrimental to its use in the food and drink industries. Since approximately 18% of the carbon dioxide produced is used for beverage carbonation, removal of these impurities from carbon dioxide is of major commercial importance. Carbon dioxide is also used in the handling and transportation of foodstuffs since the growth of bacteria can be prevented by using carbon dioxide to forestall oxidation leading to a loss of flavor. Coffee is packed under carbon dioxide, and fruits, vegetables, and cereals often are transported in an atmosphere of carbon dioxide. For all such foodstuff-related uses, the presence of contaminants which lead to an unacceptable odor or taste should be less than one milligram/kg. This application is concerned with purification of carbon dioxide for use in foodstuffs and in other areas, such as supercritical extraction and supercritical chromatography, where high purity carbon dioxide is required.
The most commonly used purification methods are treatments with potassium permanganate, potassium dichromate, or active carbon. Both potassium permanganate and potassium dichromate are active oxidizing agents, consequently scrubbing generally results in oxidation of unwanted materials. In the case of hydrogen sulfide as a contaminant, oxidation results in the formation of sulfur which is readily removed as a solid. Activated carbon has been widely used as an adsorbent for impurities from carbon dioxide. Nonetheless, the presence of residual impurities often remains a problem in providing food-grade carbon dioxide which meets the Compressed Gas Association commodity specifications or which can otherwise be used in supercritical applications. Where water is necessary to be removed from the gas, a separate drying step over alumina has sometimes been used in the past.
Although the commercial production of carbon dioxide has been ongoing for many years now, and although the purification of the gas has been the subject of many efforts, nonetheless a truly high purity carbon dioxide is expensive to process and not widely available. In particular, we have found that impurities such as water, sulfur-containing compounds, nitrogen-containing compounds, and hydrocarbons may be removed by a combination of adsorbents very effectively and very efficiently.